Feat(snowflake): add support for // comments

[georgesittas]

Fixes #1763

This also fixes a tokenizer bug related to Jinja comments ({# ... #}). In some dialects we've overridden the COMMENTS tokenizer list, excluding these delimiters, but we actually want to recognize them in all dialects.

Current behavior (note that BigQuery uses # as a comment delimiter):

>>> from sqlglot.dialects import BigQuery
>>> for tok in BigQuery.Tokenizer().tokenize("SELECT 1 {# this is a comment #}"):
...     print(tok)
...
<Token token_type: TokenType.SELECT, text: SELECT, line: 1, col: 6, start: 0, end: 5, comments: []>
<Token token_type: TokenType.NUMBER, text: 1, line: 1, col: 8, start: 7, end: 7, comments: []>
<Token token_type: TokenType.L_BRACE, text: {, line: 1, col: 10, start: 9, end: 9, comments: [' this is a comment #}']>

With the fix introduced in this PR:

>>> from sqlglot.dialects import BigQuery
>>> for tok in BigQuery.Tokenizer().tokenize("SELECT 1 {# this is a comment #}"):
...     print(tok)
...
<Token token_type: TokenType.SELECT, text: SELECT, line: 1, col: 6, start: 0, end: 5, comments: []>
<Token token_type: TokenType.NUMBER, text: 1, line: 1, col: 8, start: 7, end: 7, comments: [' this is a comment ']>